This invention relates to a method for manufacturing a shadow mask for a color picture tube, and in particular to a so-called two-step etching method.
This invention relates also to a coating apparatus for coating an etching resistant layer to be employed in the two-step etching method.
In recent years, increasingly higher definition and quality are demanded of a color picture tube for displaying characters, graphics, etc. To meet these demands, the aperture size of a shadow mask is now increasingly made smaller and the non-uniformity of aperture size of a shadow mask is also increasingly minimized.
The shadow mask can be classified generally into two kinds, i.e. a dot type shadow mask having circular apertures and a slit type shadow mask having rectangular apertures. In the case of a color display tube designed mainly for displaying characters and graphics, the dot type shadow mask is employed. While, in the case of a color picture tube designed for use in home, the slit type shadow mask is mainly employed.
The formation of apertures of a shadow mask has been conventionally performed by means of photoetching method. In the case of a shadow mask for a color display tube where high definition and high quality are demanded in particular, a two-step etching method is adopted.
In this two-step etching method, first of all, concave portions conforming to small holes facing the electron gun are formed by means of etching on one surface of a metallic thin plate. Then, an etching resistant layer is formed on the surface where aforementioned concaves conforming to small hales are formed. Thereafter, the other surface of the metallic thin plate is etched through a pattern which conforms to large holes, thereby forming through holes, each passing from a large hole side to the aforementioned concave conforming to a small hole. In this case, the diameter of each hole can be deemed to be substantially controlled by the diameter of the small hole side.
In the case of double etching method where small holes and large holes are simultaneously etched from both sides, it is difficult not only to control a side etching phenomenon which causes the etched hole size to become larger than the opening dimension of a resist, but also to precisely control the hole size since the etching of small hole proceeds even after the small hole and the large hole are communicated with each other. Whereas, in the case of the two-step etching method, the concaves of small hole side are filled with an etching resistant layer so as to prevent the small hole from being etched again in the second etching step. Therefore, the precise dimension of small hole pattern which has been formed in the first etching step can be maintained, thus making it possible to form apertures in a metal thin plate, each aperture having a diameter which is smaller the thickness of the metal thin plate.
As for the method of coating an etching resistant layer in the two-step etching method, a spray coater, a roll coater, a gravure coater or a PDN (pipe doctor nozzle) coater has been conventionally employed in the coating method. However, since apertures of much smaller size is now required to be formed in a metal thin plate in order to meet recent demand to obtain a higher definition of a display tube, these conventional methods are no more appropriate to cope with such a recent demand. For example, when an etching resistant layer is coated over hole of very small size, air bubbles tend to remain in the concaves of the small hole side, thus making the etching resistant layer defective. Therefore, if the second etching step is performed with this defective etching resistant layer, the etching of the small hole is more likely to be proceeded in the second etching step. This side etching phenomenon may become a cause of an enlargement or deformation of hole shape, of a defective hole size, of non-uniform hole size, or of the non-uniformity in quality.
Recently, an much higher definition is also demanded in a color picture tube for use in multimedia where the aperture shape is rectangular. Therefore, the aforementioned two-step etching method is also applied now to the manufacture of a shadow mask for such a color picture tube. However, in the case of rectangular apertures, the anisotropy in shape of the small hole is more prominent as compared with that of the circular apertures, and at the same time, the depth of the small hole is relatively large, so that it has been very difficult to appropriately form the etching resistant layer on the surface of a metallic thin plate where the smaller aperture are formed.